The present invention generally relates to frequency synthesizers, and particularly relates to performing direct wideband continuous phase and frequency modulation by a frequency synthesizer.
Wireless communication involves the use of radio waves to transmit data between devices and conventionally involves modulating or otherwise translating data signals to a suitable frequency for transmission. Quadrature modulation is one technique for modulating data signals. Data is conveyed using quadrature modulation by modulating the amplitude of two carrier waves in response to a data signal. The two carrier waves are approximately 90° out of phase with each other and are modulated or keyed to represent a data signal. Homodyne transmitter-based quadrature modulators comprise many components such as digital-to-analog converters and radio frequency mixers along with frequency synthesizers for generating a carrier frequency. The in-phase and quadrature signal paths of a quadrature modulator must be well matched in order to maintain acceptable signal quality.
Indirect and direct modulation of the frequency synthesizers overcome many of the limitations associated with quadrature modulators. A frequency synthesizer makes use of a Phase-Locked Loop (PLL) for performing frequency modulation. A phase comparator in a PLL compares frequencies (or phases) of two signals and produces an error signal that is proportional to the difference between the signal frequencies (or phases). The error signal, after low-pass filtering, drives a voltage-controlled oscillator (VCO), which changes its output frequency in response to the magnitude of the error signal. The VCO output is fed back to the input of the phase comparator, along with a reference frequency, producing a negative feedback loop. If the VCO output drifts, the error signal increases correspondingly, driving the PLL's output in the opposite direction so as to reduce the error. As such, the VCO output is said to be ‘locked’ to the frequency (or phase) of the reference input.
One conventional frequency synthesizer is an open-loop modulator. Open-loop modulators disable modulation just prior to a data transmission burst in order to allow a PLL component to stabilize. Once the PLL is stabilized, modulation is applied for a short time period. Open-loop modulators operate only in short bursts between tuning cycles. Further, the open-loop nature of the modulator results in a PLL free-running during bursts, thus increasing noise such as PLL phase error. In addition, the PLL conventionally requires precise calibration to ensure proper operation when used in an open-loop configuration.
Another conventional frequency synthesizer is a closed-loop modulator where the output of a VCO is fed back to an input of the synthesizer for minimizing the phase difference between the VCO output and a reference signal input. Some closed-loop frequency synthesizers perform frequency modulation by directly modulating the input of a VCO with a narrowband modulation signal. Frequency modulation is conventionally directly applied to the input of a VCO so long as the bandwidth of the modulation signal is less than the closed-loop bandwidth of the VCO, hence the term narrowband modulation. The closed-loop dynamics of a PLL do not appreciably distort modulation present in the VCO feedback signal so long as the modulation bandwidth is less than the closed-loop bandwidth of the PLL.
However, the closed-loop bandwidth of a PLL is fairly narrow in order to reduce noise such as PLL phase error, e.g., approximately 100 kHz or less in many practical applications. The closed-loop bandwidth of a PLL is mainly a function of the bandwidth of the PLL's loop filter, the current charge pump capacity of the PLL, and the divide ratio of the PLL's feedback path. Many modulation schemes use a modulation signal having a bandwidth greater than the closed-loop bandwidth of a PLL, where such modulation is referred to herein as wideband modulation. For example, wireless communication standards such as the IEEE 802.15 Bluetooth standard require modulation bandwidths greater than 100 kHz (e.g., the Bluetooth standard mandates a wideband modulation bandwidth of 500 kHz).
Signal distortion occurs when a wideband modulation signal is directly applied to a VCO, the output of which is feedback to an input of a closed-loop synthesizer. Distortion occurs when the feedback signal (including wideband modulation) is subjected to the closed-loop dynamics of a PLL. The closed-loop dynamics of a PLL attenuate the high frequency components of a wideband modulation signal and distort the modulation signal's group delay. Mainly, the limited bandwidth of the PLL's loop filter distorts the high-frequency modulation components when the VCO output is feedback to the PLL's phase comparator input.
One conventional approach for achieving wideband modulation using a closed-loop frequency synthesizer involves indirectly modulating the synthesizer's VCO. Instead of directly modulating the input of a VCO, the instantaneous divide value of a PLL's feedback path is altered responsive to a wideband modulation signal. Particularly, the nominal divide value associated with the PLL's feedback path sets the carrier frequency while instantaneous variations in the divide value cause modulation in the carrier frequency. Conventionally, a sigma-delta modulator generates a wideband modulation signal that alters the instantaneous divide value of the PLL's feedback path. As such, the wideband modulation signal is not removed from a synthesizer's feedback path. A pre-distortion filter alters the wideband modulation signal before it modulates the instantaneous divide value of a PLL's feedback path. The amplitude and group delay of the wideband modulation signal are pre-distorted so that the signal passes through the PLL without significant distortion. Ideally, the response of the pre-distortion filter is the inverse of the PLL's closed-loop response.
However, the complexity of the pre-distortion filter increases as the ratio of wideband modulation signal bandwidth to closed-loop bandwidth of the PLL increases. Further, the closed-loop dynamics of the PLL must be well-understood and accurately modeled to properly pre-distort the wideband modulation signal. The closed-loop dynamics of a PLL change as a function of fabrication process and operating parameter variation. As such, developing an accurate model of a PLL's closed loop dynamics becomes difficult when considering the wide range of process and operating parameters over which PLLs are expected to function properly.
Another conventional approach for achieving wideband modulation using a closed-loop frequency synthesizer involves injecting some components of a wideband modulation signal into one part of the synthesizer and other components into a different part of the synthesizer. Particularly, the high-frequency components directly modulate the input of a VCO and the low-frequency components alter the instantaneous divide value of the PLL's feedback path. However, equalization is needed to cancel overlap between the high and low frequency components. Otherwise, signal distortion arises. In addition, the conventional hybrid approach is adversely affected by the same operating condition variations that adversely affect operation of other conventional indirect wideband modulation schemes.